Apply letter spacing via TextStyle instead of span

Summary: This diff replaces the way we apply letter spacing at the moment for the DspText component by using a span. Instead we now leverage the TextStyle object which should match how we do that also in Litho and in [AS implementation](https://www.internalfb.com/code/fbsource/[4ffaede12082fed400fa076e2320721e6307db47]/fbandroid/java/com/instagram/fx/switcher/SwitcherCdsThemeHelper.java?lines=51).

Reviewed By: astreet

Differential Revision:
D55364499

Privacy Context Container: L1241033

fbshipit-source-id: 884148dfc3837808361962e2801478bf31493496

litho-rendercore-text/src/main/java/com/facebook/rendercore/text/TextStyle.java

@@ -284,6 +284,10 @@ public void setRoundedBackgroundColor(RoundedBackgroundProps roundedBackgroundPr
     this.roundedBackgroundProps = roundedBackgroundProps;
   }
 
+  public void setLetterSpacing(float letterSpacing) {
+    this.letterSpacing = letterSpacing;
+  }
+
   public static class RoundedBackgroundProps {
     final RectF padding;
     final float cornerRadius;